1. Field of the Invention
The present invention relates to an apparatus for representing reference images of patients, and slices to be registered in a represented reference image, for assisting the positioning of slices in preparation for a slice-by-slice data-acquisition, and to a computer software product for an apparatus of this type.
2. Description of the Prior Art
In so-called graphical slice positioning (GLP) to prepare for measurements and examinations by means of a magnetic resonance imaging apparatus, slices to be newly measured are planned on already measured images of a current patient. In this case, one or a plurality of so-called reference images of the current patient's body parts to be examined are recorded by the magnetic resonance imaging apparatus and represented on a screen. The slice-by-slice measurements to be carried out in the corresponding body part by means of the magnetic resonance imaging apparatus are planned using the represented reference image or images. In this case, the slices or slice groups are supplied to the corresponding data processing apparatus (such as e.g. a computer) by the competent doctor or the person carrying out the examination, via an input apparatus, such as e.g. a keyboard and/or a mouse, and are displayed directly in the represented reference image on the screen of the data processing apparatus. In this case, the slices or slice groups can be arbitrarily inclined and rotated in order to be able to obtain the desired images using the later measurement.
In the known methods and apparatuses for graphical slice positioning, positioning of the slices is possible in a maximum of three difference reference images of the body part to be examined. The accompanying FIGS. 2a and 2b represent, as an example, two different views as reference images of a patient's head to be examined. FIG. 2a shows, as the first reference image, a left side view of a sectional image of the head (sagittal image) and FIG. 2b shows, as the second reference image, a front view of a sectional image of the head (coronary image). The second reference image of FIG. 2b is therefore rotated through 90° relative to the first reference image of FIG. 2a. 
Also shown is a slice group formed by a number of slices 11 which were entered by a user into the corresponding data processing apparatus. The slices 11 of the slice group are doubly inclined. In the first reference image shown in FIG. 2a, the inclination of the slices as proceeding from the rear side of the head toward the face can be readily discerned. In the second reference image represented in FIG. 2b, it can be seen that the slices 11 of the slice group also are inclined in a manner proceeding from the left side of the head toward the right side of the head. From a technical standpoint, as represented for example in FIGS. 2a and 2b, the planned slices 11 of a slice group are represented in the form of sectional or projection lines in the reference images. The type of representation is distinguished as sectional lines or as projection lines by means of an automatic facility in the processing apparatus using the angular position of the planned slices relative to the respective reference image. Oblique and doubly oblique slices are represented by dashed sectional lines, as is shown for example in FIGS. 2a and 2b. Perpendicular slices are represented by a solid sectional line. In the corresponding software for image processing, the orientation of a slice is described in text form by means of a so-called history string, also called an orientation string. However, this description has a major disadvantage, namely that the sign rule for determining the direction of rotation, i.e. positive or negative, is very complicated and the risk of errors is correspondingly high.
As mentioned above, the known apparatuses and methods for representing reference images of graphic slice positioning enable only the representation of two to a maximum of three reference images. Although the slices, such as e.g. the slices 11 of the slice group represented in FIGS. 2a and 2b, can be freely displaced and rotated via a corresponding input apparatus of the assigned data processing system, it is difficult even for practiced users to correctly interpret the actual position of the slices using the depicted sectional or projection lines in the reference images. That holds true in particular when the planned slices are doubly oblique relative to the reference images used or when the slices associated with a slice group only partly penetrate the reference image. As illustrated in FIGS. 2a and 2b, doubly oblique slices arise e.g. when a user rotates an originally transverse or horizontal slice group firstly in a sagittal reference image, such as e.g. the first reference image, around the vertical sectional line 12a and then in the coronary reference image, such as e.g. the second reference image of FIG. 2b, around the other vertical sectional line 12b. 
In these cases of doubly inclined slice groups, which occur very often in cardiology and orthopedics, a high degree of spatial imagination is necessary in order to understand the actual situation behind, in some instances, the very shapeless sectional line representations, and in particular in order to assess whether the planned slices represented in the reference images actually cover the region or body part to be measured or to be examined.
Usually, according to the prior art, the user has to carry out a number of iteration steps until he or she actually arrives at the desired slice orientation required for the examination. In this case, generally an overview measurement or so-called localizer measurement is carried out initially, and then one or more further positioning processes and measurements. The result images of each measurement are in each case used for further positioning of the slices. However, since the reference images of the localizer measurements in each case represent only one sectional view and a maximum of three reference images can be used for positioning the slices, often a number of attempts have to be made one after the other before the region to be examined is represented ideally on the resultant images of a measurement, i.e. before the optimum slice position and orientation has been found.